ML20239A956
| ML20239A956 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 08/26/2020 |
| From: | Lochbaum D Pacific Gas & Electric Co |
| To: | Division of Operating Reactor Licensing |
| Lee S, 301-415-3158 | |
| Shared Package | |
| ML20239A951 | List: |
| References | |
| EPID L-2020-LLA-0176 | |
| Download: ML20239A956 (11) | |
Text
EVALUATION OF THE POTENTIAL PIPING DEGRADATION SAFETY CONCERN ON THE AUXILIARY FEEDWATER SYSTEM AT DIABLO CANYON UNIT 1 AN ANALYSIS PERFORMED IN SUPPORT OF THE COMMENTS SUBMITTED TO THE NRC ON BEHALF OF THE MOTHERS FOR PEACE OF SAN LUIS OBISPO DAVID LOCHBAUM AUGUST 26, 2020
Diablo Canyon Auxiliary Feedwater (AFW) System Design During normal reactor operation, three loops transfer the heat generated by the reactor core to the Pacific Ocean, producing electricity along the way. The primary loop, shown in red on the right of Figure 1, consists of the reactor vessel, the steam generator (SGs), the reactor coolant pumps (RCPs), the pressurizer (PZR) and connecting piping. Water heated flowing through the reactor core flows to the steam generators. As this water flows insider thousands of metal tubes inside the steam generators, heat conducts through the metal walls to boil water in the secondary loop. The cooled water leaves the steam generators to be pumped back to the reactor vessel.
Figure 1 Source: U.S. Nuclear Regulatory Commission, Westinghouse Technology Manual, 2020 (ML20057E160)
The secondary loop, also shown across the top of Figure 1, consists of the four steam generators, the main turbine/generator, the condenser, the condensate pumps, the condensate and feedwater heaters, and the feedwater pumps. Steam produced in the steam generators flows to the turbine to spin the generator and make electricity. Steam exhausts from the turbine into the condenser. Thousands of metal tubes pass ocean water through the condenser to cool the steam down and convert it back into water. The condensate and feedwater pumps draw water from the condenser and recycles it to the steam generators.
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The tertiary loop features pumps (not shown in Figure 1) drawing water from the Pacific Ocean and sending it through metal tubes within the condenser. The water, warmed up to 30°F flowing through the condenser, flows back into the ocean.
The feedwater system is not designed to withstand an earthquake or to operate if the plants connection to the offsite power grid is lost. The Auxiliary Feedwater (AFW) system is its emergency backup. Three AFW pumps can transfer water from either of two large storage tanks to the steam generators so heat from the reactor continues to get removed.
The AFW system, shown in the upper right center of Figure 1, also performs this vital role in case of an accident. Should a primary loop pipe break and drain reactor coolant water into the containment building, the emergency systems shown within the Auxiliary Building in the lower center of Figure 1 will automatically start and provide makeup water to the reactor vessel. The AFW system supplements this makeup cooling water function by continuing to remove heat from the primary loop via the steam generators.
Figure 2 Source: U.S. Nuclear Regulatory Commission, Diablo Canyon Unit 2 Plant Information Book, August 12, 1994 August 26, 2020 Page 3
Figure 2 is a schematic of the AFW system at Diablo Canyon. Each of its two reactors has its own AFW system like shown here. The AFW system features two motor-driven pumps and one turbine-driven pump. The motor-driven pumps are powered from the normal electrical distribution system and can be powered from the onsite emergency diesel generator should the normal power supply be unavailable. The turbine-driven AFW pump uses steam produced by the decay heat produced by the shut down reactor core. The control system for this turbine-driven AFW pump is powered from onsite battery banks allowing this pump to function even if normal and emergency diesel generator power supplies are unavailable.
The AFW pumps draw water from the Condensate Storage Tank (CST) or the Raw Water Reservoir.
Another source of water can be obtained by manually opening valves to allow the AFW pumps to draw water from the Firewater Tank.
The AFW system piping downstream of the pumps is cross-connected. There is a recirculation path back to the CST allowing the AFW pumps to be tested while the reactor operates without supplying water to the steam generators. Normally closed valves in the downstream piping route the flow from AFW Pump 2 to Steam Generators 1 and 2 and the flow from AFW Pump 3 to Steam Generators 3 and 4. These valves can be manually opened to allow these pumps to supply makeup water to all four steam generators. The flow from turbine-driven AFW pump 1 is normally routed to all four steam generators.
The downstream piping contains Level Control Valves (LCVs) that regulate the AFW flow rate to the steam generators to maintain the water level inside the steam generators at the Goldilocks level -
neither too high nor too low, but just right. The LCVs are throttled partially open. If water level drops too low in a steam generator, its LCVs opens wider to increase the AFW flow rate and restore the desired water level. If the water level rises too high in a steam generator, its LCVs closes more to decrease the AFW flow rate and bring the water level back within the desired band.
The AFW system serves a safety function during many transient and accident scenarios. About the only emergency conditions in which the AFW system has little or no role to play involve medium or large sized loss of coolant accidents. If a medium or large pipe in the primary loop were to break, water would flow from its broken ends at such a high rate that while the emergency systems on the left can provide adequate makeup flow to the vessel to keep the reactor core covered and cooled, they may be unable to keep the entire primary loop filled with water. If primary loop water does not flow through the steam generators, the AFW system cannot remove its heat.
In medium and large sized loss of coolant accidents, the emergency systems will provide makeup water to the reactor vessel. The containment spray system, with nozzles mounted to the roof of the containment building car-wash style, will spray water into the containment to cool the primary loop water now pooling on the containment floor after flowing from the broken pipe ends.
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Diablo Canyon Auxiliary Feedwater (AFW) System Licensing Requirements The NRC issued operating licenses for Diablo Canyon Units 1 and 2. An appendix to the operating license called the Technical Specifications establish what conditions are required when for the reactor to be operated. Technical Specification Limiting Condition for Operation (LCO) 3.7.5 (see Figure 3) governs the AFW system at Diablo Canyon.
Figure 3 August 26, 2020 Page 5
Source: U.S. Nuclear Regulatory Commission, Diablo Canyon Nuclear Power Plant, Unit 1 Facility Operating License, January 8, 2008.
These licensing requirements apply when the reactor is in MODES 1, 2, and 3 and conditionally when it is in MODE 4. (No steam is produced in MODES 5 and 6, so no AFW system is needed.) In MODES 1, 2 and 3, three AFW trains (i.e., water supply sources, AFW pumps, controls, piping, and valves) are required to be operable. The MODES are also defined by the Technical Specifications (see Figure 4).
Figure 4 Source: U.S. Nuclear Regulatory Commission, Diablo Canyon Nuclear Power Plant, Unit 1 Facility Operating License, January 8, 2008.
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The ACTION statements define what to do when an LCO is not met. For example, ACTION B requires an inoperable AFW train to be restored to service within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. If a single AFW train cannot be restored within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> or if a second AFW train is inoperable concurrently, ACTION C requires the reactor to be placed in MODE 3 within six hours and in MODE 4 within 18 hours2.083333e-4 days <br />0.005 hours <br />2.97619e-5 weeks <br />6.849e-6 months <br />. If all three AFW trains are inoperable concurrently, ACTION D requires the reactor to be shut down immediately.
If a single AFW train is inoperable and is not restored to operable within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, the six-hour shutdown clock in ACTION C.1 starts. If the AFW train is restored to operable before the six-hour clock times out, LCO 3.7.5 is satisfied (assuming the other two AFW trains are operable) and ACTIONS B and C no longer apply - in other words, the reactor may continue operating because LCO 3.7.5 is satisfied.
Diablo Canyon AFW Licensing Requirements Compared to Historical Standards In May 1976, the NRC issued standardized Technical Specifications for pressurized water reactors PWRs) designed by Westinghouse.1 The intent was to serve as a template for future reactor operating licenses issued by the agency. It contained only the equivalent of ACTION B from Diablo Canyons LCO 3.7.5:
With one auxiliary feedwater pump inoperable, restore at least three auxiliary feedwater pumps (two capable of being power from separate emergency buses and one capable of being powered by an OPERABLE steam supply system) to OPERABLE status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.
The NRC periodically revised its Standard Technical Specifications for Westinghouse PWRs. Over the ensuing decades and revisions, the 72-hour time limit for one AFW train being inoperable remained in place and was supplemented by Diablo Canyon-like ACTIONS C and D for multiple AFW trains being inoperable. For example, Revision 4 of the NRCs Westinghouse Standard Technical Specification released in April 2012 mirrors the content of Diablo Canyons LCO 3.7.5 72-hour limit on one AFW train inoperable, 6-hour limit on two AFW trains inoperable, and immediate shutdown when all three AFW trains are inoperable.2 In other words, the 72-hour limit on operating a reactor with an inoperable AFW train has been in place at Diablo Canyon and dozens of U.S. PWRs for decades. The AFW licensing requirements for Diablo Canyon are not stricter or more conservative than elsewhere they are exactly the same as they have been everywhere for decades.
1 U.S Nuclear Regulatory Commission, Standardized Technical Specifications for Westinghouse PWRS, NUREG-0452, May 15, 1976 (ML17266A005) 2 U.S. Nuclear Regulatory Commission, Standard Technical Specifications - Westinghouse Plants, Revision 4.0, Vol. 1, Specifications, April 2012 (ML1200A222)
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Auxiliary Feedwater (AFW) System Licensing Precedents The 72-hour limit on reactors operating with one AFW has been in place at U.S. PWRs for decades.
There have been occasions when owners sought NRCs permission to continue operating reactors longer than 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> with an AFW train inoperable.3 An extensive search of NRCs publicly available records identified the following four occasions listed in reverse chronological order (i.e., newest first):
Indian Point Unit 3, 2009/2010 In December 2009, Entergy applied to the NRC for an exigent license amendment to allow Indian Point Unit 3 to continue operating for up to 106 hours0.00123 days <br />0.0294 hours <br />1.752645e-4 weeks <br />4.0333e-5 months <br /> instead of the current 72-hour limit while it implemented repairs to AFW pump 32.4 On January 5, 2010, the NRC denied Entergys exigent license amendment request for the following reason:5 The Nuclear Regulatory Commission (NRC) staff has reviewed the submittals and has determined that there is insufficient justification to use the exigent provisions of 10 CFR 50.91. The NRC staff finds that there is not a sufficient need to act quickly such that the normal public notice period should be reduced. As stated in the amendment and supplements referenced above, you have determined that the turbine-driven auxiliary feedwater pump is currently operable per the IP3 Technical Specifications, and capable of performing its safety mission.
Because AFW pump 32 on Indian Point Unit 3 was operable, there was no pressing need for NRC to process an expedited license amendment. The lack of urgency afforded Entergy the opportunity to pursue a license amendment via the routine process with normal public notifications.
Entergy submitted a license amendment request to the NRC on January 11, 2010, seeking a one-time extension of the 72-hour limit to 106 hours0.00123 days <br />0.0294 hours <br />1.752645e-4 weeks <br />4.0333e-5 months <br /> so workers could remove AFW pump 32 from service and implement upgrades.6 Before the NRC could decide to approve or deny the routine license amendment request, Entergy withdrew the request. Turns out that workers were able to modify AFW pump 32 within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, negating the need for an extension of the out of service time.7 3
The author thanks David Weisman of the Alliance For Nuclear Responsibility for suggesting that AFW precedents be examined to their applicability to the current Diablo Canyon matter.
4 Entergy Nuclear Northeast, Proposed Exigent License Amendment Regarding One Time Extension of the Auxiliary Boiler Feedwater Pump Allowed Outage Time - Indian Point Unit Number 3, December 15, 2009 (ML093400595) 5 U.S. Nuclear Regulatory Commission, Indian Point Nuclear Generating Unit No. 3 - Determination on Exigent Circumstances for License Amendment Request, January 5, 2010 (ML100050260) 6 Energy Nuclear Northeast, Proposed License Amendment Regarding One Time Extension of the Auxiliary Boiler Feedwater Pump Allowed Outage Time - Indian Point Unit Number 3, January 11, 2010 (ML100670176) 7 Entergy Nuclear Northeast, Withdrawal of Proposed License Amendment Regarding One Time Extension of the Auxiliary Boiler Feedwater Pump Allowed Outage Time - Indian Point Unit Nuclear 3, November 17, 2010 (ML103410153)
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Catawba Unit 1, 2008 The NRC issued an emergency license amendment on July 15, 2008, that allowed Catawba Unit 1 to continue operating for up to 9 days while workers repaired a service water pump that failed during a test run. This service water pump supported AFW train 1B, so its failure resulted in AFW train 1B being declared inoperable, starting the 72-hour restoration or shutdown clock. 8 South Texas Project Unit 1, 2006 The NRC issued a Notice of Enforcement Discretion (NOED) on December 20, 2006, allowing South Texas Project Unit 1 to operate for up to 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> longer than the 72-hour limit for one AFW train being inoperable. An in-plant power transient due to a ground fault in the plants electrical distribution system caused a capacitor to fail in the power supply for the instrumentation and control circuits for turbine-driven AFW train D.9 Palisades, 2000 Consumers Energy applied for an exigent license amendment to allow its Palisades reactor to restart with turbine-driven AFW Pump P-B inoperable. The underground backup steam supply line to this AFW pump developed a leak due to external corrosion. The safety analyses for Palisades did not take credit for this backup steam supply to the turbine-driven AFW pump. The license amendment request sought to remove mention of this backup steam supply line and rely solely on the primary steam supply line exclusively credited in the safety studies.10 On March 14, 2000, the NRC issued an exigent license amendment that enabled Palisades to operate without the backup steam supply line to the turbine-driven AFW pump. 11 These precedents reveal that the NRC has granted relief from the 72-hour time limit on reactor operation with one AFW train inoperable when an AFW train was inoperable. On Catawba Unit 1 and South Texas Project Unit 1, an AFW train was declared inoperable and could not be restored within the 72-hour limit. Consequently, the reactor would have had to be shut down but for the relief granted by the NRC (an emergency license amendment for Catawba and a Notice of Enforcement Discretion for South Texas Project). An inoperable AFW train would have prevented Palisades from starting up until the backup steam supply to its turbine-driven AFW pump but for the relief granted by the NRC (an exigent license amendment).
The precedents further reveal that the NRC has denied relief from the 72-hour time limit on reactor operating with one AFW train inoperable when no AFW train was inoperable. On Indian Point Unit 3, the plant owner contended and the NRC concurred that AFW pump 32 was operable. Because all required AFW trains were operable, the NRC determined that the owner could pursue a license amendment request via the normal process if it sought to extend the 72-hour limit.
8 U.S. Nuclear Regulatory Commission, Catawba Nuclear Station, Unit 1, Issuance of Emergency Amendment Regarding One-Time Extension of the Auxiliary Feedwater System and the Containment Spray System Allowed Outage Time, July 15, 2008 (ML081980347) 9 U.S. Nuclear Regulatory Commission, Notice of Enforcement Discretion for South Texas Project Nuclear Operating Company Regarding South Texas Project Unit 1, December 20, 2006 (ML063540518) 10 Consumers Energy, Palisades Plant - Technical Specifications Change Request - Auxiliary Feedwater, February 18, 2000 (ML003686886) 11 U.S. Nuclear Regulatory Commission, Palisades Plant - Issuance of Amendment Re: Backup Steam Supply or Turbine-Driven Auxiliary Feedwater Pump P-8B, March 14, 2000 (ML003692656)
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Diablo Canyon Auxiliary Feedwater (AFW) System Licensing Conundrum Pacific Gas and Electric Company submitted an exigent license amendment request to the NRC on August 12, 2020, seeking to extend the 72-hour limit that Unit 1 could operate with one AFW train inoperable to a 7-day limit.12 PG&E informed the NRC that:
On July 23, 2020, with DCPP Unit 2 still in Mode 3, a 3.9 gallons per minute calculated through-wall leak was observed coming out of the elbow just downstream of Valve LCV-111 in the discharge line for Unit 2 AFW Pumps 2-1 and 2-2 to SG 2-2. The unit was transitioned to Mode 4 in accordance with DCPP TS 3.7.5, Required Action D.2. Repairs were made to the AFW piping while shutdown prior to returning Unit 2 to power operation.
An Extent of Condition (EOC) Investigation performed for the Unit 2 AFW piping leak identified no additional leaks. However, six additional locations were identified in the Unit 2 AFW system where repairs were required because pipe wall thickness did not meet minimum ASME code requirements. The repairs were completed and inspected, and the affected AFW trains were returned to operable status on July 31, 2020.
With the AFW system piping corrosion problem found and fixed on Unit 2, PG&E sought the exigent license amendment request to facilitate its fixing the same problem it suspected existed on Unit 1.
If PG&E expected that it would find no piping degradation when it inspected the Unit 1 AFW piping, no repairs would be necessary and the current 72-hour limit would not be relevant.
If PG&E expected that it would find piping degradation when it inspected the Unit 1 AFW piping, repairs might take longer than the current 72-hour limit. To hedge against this situation, PG&E submitted an exigent license amendment request seeking additional time to implement the repairs that inspections might reveal to be necessary.
But the NRC has granted relief of the 72-hour limit in the past only when an AFW train was inoperable.
The NRC denied Entergys exigent license amendment request for repairs to AFW pump 32 because it had not declared the pump inoperable. With AFW pump 32 operable, Entergy could (and did) submit a regulator license amendment request to the NRC seeking a relaxation of the 72-hour limit for it to repair AFW pump 32.
Absent an AFW train being inoperable, the NRC cannot grant relief via an emergency license amendment, an exigent license amendment, or a Notice of Enforcement Discretion. Because PG&E has not declared any of the AFW trains on Diablo Canyon Unit 1 inoperable, none of these relief options are available.
But if PG&E declares an AFW train inoperable in order to avail itself of the emergency license amendment, exigent license amendment, or Notice of Enforcement Discretion licensing options, it would open itself to charges of deliberate misconduct. For it had reason to believe no later than August 12, 2020, and almost certainly two weeks before then that the Unit 1 AFW piping degradation rendered one or more of the AFW trains inoperable. Its exigent license amendment request is compelling prima facie evidence of this belief. Declaring now that a Unit 1 AFW train is inoperable is tantamount to confessing to have willfully operating Unit 1 longer than permitted by LCO 3.7.5.
12 Paula Gerfen, Site Vice President, Pacific Gas and Electric Company, Diablo Canyon Units 1 and 2, License Amendment Request 20-01, Exigent Request for Revision to Technical Specification 3.7.5.
Auxiliary Feedwater System, August 12, 2020 (ML20225A303).
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Diablo Canyon Auxiliary Feedwater (AFW) System Licensing Conundrum Exit Plan The second-best plan13 for PG&E to exit its Unit 1 AFW system licensing conundrum would be to immediately inspect the Unit 1 AFW system piping. The reactor need not be shut down to perform these inspections.
Inspection all Unit 1 AFW system piping would define the scope of the problem, ranging from no problem if no or little piping degradation is identified to major problem if piping segments in all three AFW trains are discovered to have unacceptable levels of degradation. Once the scope of the problem gets defined, the appropriate solution becomes readily evident.
If no problem is found, no solution is needed.
If only one AFW train is found impaired, an exigent license amendment request or Notice of Enforcement Discretion might be a viable solution.
If multiple AFW trains are found impaired, shutdown until the AFW systems problems are corrected would be appropriate.
13 The best plan would have entailed conducting the inspections of the Unit 1 AFW system piping as soon as possible after July 31, 2020, when workers completed replacement of degraded piping segments in the Unit 2 AFW system and restored all three AFW trains to operable status Those skilled and experienced workers could have pivoted to repeat the inspections on Unit 1. Instead, PG&E indulged in some foot-dragging and clock-unwatching.
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